Beyond the Single Storm

May 29, 2018
Developments in rainwater harvesting system design

About the author: Sarah Lawson, Ph.D., is water specialist for Rainwater Management Solutions. Lawson can be reached at [email protected] or 434.293.4485.


Two years ago, I was working with a team of designers on a rainwater harvesting system for a large municipal building. While calculating the return on investment, the group asked the civil engineer how other storm water management practices had been downsized because of the rainwater harvesting system. The civil engineer immediately replied that the rainwater harvesting system could not be included in the storm water management plan because “it would sometimes be full.”

Stumbling Block

Determining the reduction in runoff from a rainwater harvesting system is a stumbling block for many designers and regulators. The typical solution to the tank-will-sometimes-fill-up problem is to empty the tank over a predetermined number of days (often 3 to 5) in an approach similar to a traditional detention pond, which also might fill to capacity.

Draining the storage tank in a few days to make space for the next storm is counterproductive and removes two of the great benefits of rainwater harvesting: a decrease in the total volume of runoff and a reduction in reliance on the potable water supply. Instead of viewing rainwater saved for use as an impediment to future storm storage, a gallon of rainwater used should be seen as a gallon of runoff prevented.

Virginia Leads the Way

Virginia has created an approach to incorporating rainwater harvesting into storm water management plans that recognizes the practice’s benefits. The storm water credit is calculated from a time series model using 30 years of daily rainfall data. The water level in the storage tank is calculated for each day based on the previous day’s water level, the input from measured rainfall data, drawdown from the estimated water use demand and overflow from the tank if it reaches capacity (see Figure 1).

Figure 1

The model results are then summarized to show how effective the rainwater harvesting system is at preventing runoff for water quality—in this case, from a 1-in. or less storm. This is achieved by calculating the percentage of the total volume of precipitation from rain events that is captured and reused. For example, at a building with a 10,000-sq-ft footprint in Richmond, Va., storms with less than 1 in. of total precipitation in 24 hours would generate an average of 158,000 gal of runoff per year. If rainwater was harvested from the roof, stored in a 10,000-gal cistern and used to flush toilets (800 gal per day Monday through Friday), the same storms would generate 18,000 gal of runoff per year.

The runoff reduction credit is then calculated from the modeled reduction in runoff. This reduction in runoff is directly related to beneficial use of the harvested rainwater (see Figure 2). This approach also recognizes that precipitation typically occurs as part of a larger weather system, not an isolated 24-hour event.

Figure 2

A belief that storm water design must move beyond single storm events is growing across the industry. Rainwater harvesting is one prominent example that real rainfall patterns—not one-time events—must be considered. Rainwater harvesting brings together the issues of water supply and runoff, forcing a look at the bigger picture.

For more information on Virginia’s best management practice (BMP) guidelines for rainwater harvesting, visit the Virginia Stormwater BMP Clearinghouse at